I have this trouble times 3. Due to my unusual rear hub, it is bolted on with a different sized bolt on each side, plus an extra piece that is a 3rd size. I regularly travel with a set of Craftsmen Robogrip pliers. Not too heavy, not too light, but they fit multiple nuts and can be used in other ways as well. It is not the perfect tool for the job, but it works, and that's good enough for a seldom-used traveling tool. Still, having the properly-sized wrench can make things a lot easier. There are a few multi tools that incorporate a standard, 15mm wrench into it, so that might be worth investigating.

While changing a rear flat on my bike is a hassle, I do feel like it has it's advantages in terms of security. If I'm concerned about theft, I always have to make sure the frame and the front wheel are locked to something. Someone has to be really determined if they want to steal my back wheel, even if it's not locked to anything.